Window for radiation detectors and the like

ABSTRACT

An improved x- and gamma- radiation and particle transparent window for the environment-controlling enclosure of various types of radiation and particle detectors is provided by a special graphite foil of a thickness of from about 0.1 to 1 mil. The graphite must have very parallel hexagonal planes with a mosiac spread no greater than 5* to have the necessary strength in thin sections to support one atmosphere or more of pressure. Such graphite is formed by hot-pressing and annealing pyrolytically deposited graphite and thereafter stripping off layers of sufficient thickness to form the window.

United States Patent [191 Sparks, Jr. et al.

[ 1 WINDOW FOR RADIATION DETECTORS AND THE LIKE [75] Inventors: CullieJ. Sparks, Jr., Oak Ridge; Jack C. Ogle, Knoxville, both of Tenn.

[73] Assignee: The United States of America as represented by the UnitedStates Energy, Research and Development Administration, Washington, DC.

[22] Filed: Sept. 4, 1974 21 Appl. No.: 503,121

[52] US. Cl. 250/389; 250/370; 250/510; 313/93 [51] Int. Cl. H01J 39/26[58] Field of Search 250/389, 526, 510, 505, 250/370; 313/93 [56]References Cited UNITED STATES PATENTS 2,461,254 2/1949 Bassett 250/5102,552,723 5/1951 Koury 313/93 2,574,000 11/1951 Victoreen 313/93 Oct.28, 1975 2,577,106 12/1951 Coleman 250/389 2,596,080 5/1952 Raper et a1.313/93 2,837,677 6/1958 Hendee et 31.... 250/374 3,576,439 4/1971Figueroa 250/370 3,742,230 6/1973 Spears et a1. 250/510 PrimaryExaminer-l-larold A. Dixon Attorney, Agent, or Firm-Dean E. Carlson;David S. Zachry; David E. Breeden [57] ABSTRACT An improved xandgammaradiation and particle transparent window for theenvironment-controlling enclosure of various types of radiation andparticle detectors is provided by a special graphite foil of a thicknessof from about 0.1 to 1 mil. The graphite must have very parallelhexagonal planes with a mosiac spread no greater than 5 to have thenecessary strength in thin sections to support one atmosphere or more ofpressure. Such graphite ,is formed by hotpressing and annealingpyrolytically deposited graphite and thereafter stripping off layers ofsufficient thickness to form the window.

2 Claims, 2 Drawing Figures US, Patent Oct. 28, 1975 WINDOW FORRADIATIONDETECTORS AND THE LIKE This invention was made during the course of, orunder, a contract with the United States Atomic Energy Commission.

BACKGROUND OF THE INVENTION This invention relates generally to ionizingradiation detectors and more specifically to radiation detectors of thetype having an environment-controlling enclosure with a radiationtransparent window therein.

many types of detectors for ionizing radiation, such as neutrons,charged particles, gammas or x-rays, are provided with an enclosurewhereby the active detector element may be maintained in a vacuum or adesired gaseous environment. However, in order for the ionizingradiation or particle to reach the detecting element, a window must beprovided which will admit the radiation or particle and, at the sametime, maintain the environment around the detector which may include theexclusion of light or infrared photons for certain type detectors suchas the silicon diode.

A typical example of this construction is a solid state detector forx-rays. A Si(Li) detector is mounted in a housing so as to becryogenically cooled, and a vacuum of about Torr is maintained aroundthe detecting element. A thin (usually 1 mil) beryllium foil covers acircular window opening in the housing having a diameter of about 0.25to 0.5 in. Beryllium has been utilized in the prior art because its lowatomic number and high tensile strength gave the better relativetransparency, compared to other materials, to the radiation to bedetected, especially soft x-rays. Commercial beryllium after hot rollingto thin foils suitable for detector windows is normally about 98% pure;some of the principal impurities are iron, nickel and copper. However,if as much as 0.3 wt Cu is present, the absorption by the foil isincreased by a factor of 2 or more for low energy radiations," i.e., themost strongly absorbed radiation and, therefore, less intense radiationis best benefited by reduction of impurities in the foils. This is thetype of radiation utilized for x-ray fluorescence analysis.

Furthermore, the mounting of thin Be windows by brazing or glueing isgenerally considered to be difficult and failures in excess of 80% canoccur during fabrication because of the brittleness of the beryllium andsmall holes. The l-mil foil is most generally used but thicknesses to0.5-mil of beryllium have been used successfully to withstand a oneatmosphere of pressure differential but with an increased percentage offailures.

SUMMARY OF THE INVENTION In view of the above, it is one object of thesubject development to provide a thinner window for radiation detectorshaving a lower absorption for the radiation, particularly low energyradiation.

It is another object to provide a window that may be fabricated withgreater success than that of the prior art thus reducing the cost.

Other objects and many of the attendantadvantages of the presentinvention will become apparent from the following detailed description,taken in conjunction with the drawings, of a thin graphite window, of athickness of about 0. l-mil, wherein the hexagonal (basal) crystalplanes are substantially parallel to the surface of the foil providingan extemely high tensile strength.

would be expected to be necessary to withstand the BRIEF DESCRIPTION OFTHE DRAWINGS DETAILED DESCRIPTION It has been known in the prior artthat a graphite body, having substantially parallel basal planes, can beprepared by hot-pressing and annealing pyrolyticallydeposited graphite.These bodies have been utilized, for example, as x-ray and neutrondiffraction targets,

etc. The planes are about 3.35 A apart and have a mosaic spread of nomore than 5 but generally 1 or less.

Because carbon is only a little more absorbing toward radiation (AtomicNo. 6 versus 4) than is Be, and can be prepared in very pure form, thinlayers of the highly oriented graphite were investigated as a window fordetectors. Although a thickness of greater than 6 mils pressuredifierential based upon published strength data, theoretical strengthcalculations show that foils as thin as A could possibly support oneatmosphere of pressure over a hole of 0.5 in. diameter. Graphite foilsof the order of l-mil were first investigated in order to compare themwith l-mil Be foils.

These foils were prepared by lifting layers of the graphite from apressed body in a manner similar to separating layers of mica. Forexample, an adhesive tape may be applied along an edge of the surfacefollowed by a careful peeling up of the layer. The windowis trimmed outof the portion not in contact with the tape and either clamped betweenflanges or glued to a flange for use inthe detector system. The simplepeeling method generally produces a foil of about l-mil thickness. Toprepare thinner foils, a parting agent, such as water, is applied to theedge of the pressed body prior to a peeling step. This agent apparentlyworks along the planes of the graphite body and foils of 0.1 to 0.2 milsare normally produced.

Graphite foils of various thickness were clamped between two flangeshaving a 0.25 in. diameter opening, using Teflon washers on eachsurface. These were subjected to a vacuum of up to 10" Torr on one side.Foils as thin as 0.1-mil withstood this pressure differential.

Referring now to FIG. 1 there is shown a radiation detector includingthe graphite foil window 5 of the present invention sealably mountedover an opening in the detector housing 7 by means of an annular flange8. This detector housing 7 encloses a solid state detecting element,such as a silicon diode 9. Typically, the diode 9 is mounted at the backof an annular shield 11 which has a central opening smaller than thesensitive detection area to prevent deposition of energy in theperipheral regions of the diode where charge collection is incompletethus reducing the energy of the pulse and contribution to thebackground.

The shield 11 and diode 9 assembly is mounted on insulators 13 withinthe housing 7 and the output leads 15 from the diode exit the housingthrough a sealed opening 17 in the housing back. A vacuum lineconnection 19 is provided to evacuate the housing 7 during radiationdetection.

A detector of this type may be used with a radiation collimator (notshown) forward of the window 5 and the window is typically about 0.5inch in diameter.

Referring now to FIG. 2 there is shown a detector of the gasfilledproportional counter type which has a housing 7 and graphite window 5similar to the embodiment shown in FIG. 1. The detector anode electrodeis provided by means of a circular wire grid 21 and the cathodeelectrode is formed by an electrically conductive plate 23. The grid 21and plate 23 are spaced apart and supported within the housing 7 bymeans of insulator mounts 25. The electrical connections to theelectrodes are made via a sealed opening 27 in the housing 7. Typically,a counter of this type is filled with a pressurized ionizable gas to apressure of about 1 atmosphere.

The theoretical transmission of the characteristic xrays of severalelements has been compared with that for Be windows. These calculationsindicate that the transmission. of a 0.1-mil graphite foil is about thesame as for a 0.33 mil high purity beryllium foil. Thus, a 0.1- milgraphite window may transmit about three times as much x-radiation atenergies less that l KeV as a l-mil high purity 99.9%) Be foil. Thetable below shows the comparison of the fraction of various x-radiationenergy transmitted (P,) to that incident (P for a 0.1 mil thick graphitewindow and a l-mil thick Be window using known absorption coefiicients.

Since a thin Be foil has about 2% impurities, in actual practice the0.1-mil graphite is about 10 times more transparent and does not exhibitthe many absorption edges which arise from the impurities in Be andwhich cause difficulties in operation as each absorption edge must bedetermined.

Other calculations have been made of the apparent and expected strengthof the graphite foils. The calculations indicate the strength in theplane of the basal sheets to be in excess of 2000 Kg/cm considerablyhigher than published values for hot-pressed pyrolytic graphite but nearthe values published for graphite fibers. This result predicts that verythin foils of a thickness of 0.1-mil will support one atmosphere ofpressure.

Accordingly it will be seen that an improved ionizing radiation detectorwindow has been provided by employing a graphite foil produced frompyrolytically deposited hot-pressed and annealed graphite.

What is claimed is:

1. In an ionizing radiation detector including a housing having aradiation pervious window therein, the improvement comprising: saidwindow being a graphite foil formed from pyrolytically-deposited hotpressed and annealed graphite.

2. The detector as set forth in claim 1 wherein said housing having anatmosphere therein at a pressure producing a pressure differentialacross said window of about 1 atmosphere and wherein said graphite foilwindow has a uniform thickness in the range of from about 0.1 to 1 mil.

1. IN AN IONIZING RADIATION DETECTOR INCLUDING A HOUSING HAVING ARADIATION PERVIOUS WINDOW THEREIN, THE IMPROVEMENT COMPRISING SAIDWINDOW BEING A GRAPHITE FOIL FORMED FROM PYROLYTICALLY-DEPOSITED HOTPRESSED AND ANNEALED GRAPHITE.
 2. The detector as set forth in claim 1wherein said housing having an atmosphere therein at a pressureproducing a pressure differential across said window of about 1atmosphere and wherein said graphite foil window has a uniform thicknessin the range of from about 0.1 to 1 mil.